Speech coders and decoders are known in the art. Some speech coders convert analog voice samples into digitized representations, and subsequently represent the spectral speech information through use of linear predictive coding. Other speech coders improve upon ordinary linear predictive coding (LPC) techniques by providing an excitation signal that is related to the original voice signal.
U.S. Pat. No. 4,817,157 describes a digital speech coder and decoder having an improved vector excitation source wherein a codebook of codebook excitation vectors is accessed to select a codebook excitation signal that best fits the available information, and is used to provide a synthesized speech signal from an LPC filter that closely represents the original.
Once the synthesized speech signal has been developed, various post-LPC filters are often used to further condition the signal. One such filter is an adaptive spectral postfilter (7hich is typically intended to enhance the perceptual quality of the synthetic speech), and another is a post emphasis filter (7hich contributes brightness to the synthetic speech result).
An adaptive spectral postfilter is typically of the general form: ##EQU1##
The denominator term in the above postfilter representation emphasizes the formants in the synthetic signal spectrum, while attenuating the spectral valleys. (In the two extremes, setting .nu.=0 results in an all-pass filter, while setting .nu.=1 results in a denominator term that is the same as the associated LPC filter.) The numerator term attempts to cancel the general spectral shape introduced by the denominator. In prior art applications, .nu. is often set to about 0.8, and .eta. to about 0.5.
In practice, the numerator polynomial is only partially successful in tracking the spectral shape of the denominator (in effect, the spectral characteristic of the filter tilts with time), and that discrepancy typically manifests itself as a time varying modulation of the postfiltered speech brightness.
Accordingly, a need exists for a method of postfiltering synthesized speech that will both enhance the perceptual quality of the synthetic speech, while simultaneously minimizing detrimental impact on speech brightness. Preferably, speech brightness itself will be better controlled as well.